A PON (Passive Optical Network) is an optical network constituted by a single OLT (Optical Line Terminal) and a plurality of ONUs (Optical Network Unit). In the PON, the OLT and each of the ONUs are connected to each other via a trunk cable and a distribution cable. One end of the trunk cable is connected to the OLT, and the other end of the trunk cable is connected to an optical splitter (optical coupler). One end of the distribution cable is connected to the optical splitter, and the other end of the distribution cable is connected to one of the ONUs.
In the PON, the optical splitter creates a burst optical signal by combining packets received from respective ONUs, and the OLT receives such a burst optical signal. The burst optical signal has a high strength during a period of time in which an optical signal corresponding to a packet transmitted through a path causing a small loss is inputted, and a low strength during a period of time in which an optical signal corresponding to a packet transmitted through a path causing a large loss is inputted. The OLT monitors power of such a burst optical signal for each period of time in which an optical signal is received. Therefore, a failure occurring in a path between the OLT and each of the ONUs can be detected. For this reason, an optical receiver (also called “optical transceiver” in some cases) of the OLT is required to have a power monitor function, that is, a function of outputting a monitor signal indicating power of a received optical signal.
FIG. 6 is a circuit diagram illustrating an arrangement of a conventional optical receiver 6 including a monitoring circuit 60 having such a power monitor function. The optical receiver 6 includes a photodiode PD for converting a received optical signal into a current signal, a transimpedance amplifier TIA for converting the current signal obtained by the photodiode PD into a voltage signal, and a monitoring circuit 60 for generating a monitor signal indicating a strength of the current signal obtained by the photodiode PD, i.e. power of the received optical signal (see FIG. 6).
The monitoring circuit 60 includes: a current mirror circuit 61 having an input point 61a connected to the photodiode PD; and a resistor Rm, one end of which is connected to an output point 61b of the current mirror circuit 61, and the other end of which is connected to ground. The monitoring circuit 60 outputs, to an external device, a monitor signal whose value is an electric potential Vm at the output point 61b of the current mirror circuit 61.
In a case where the current mirror circuit 61 has a current mirror ratio of n:1, a monitor current “Im=Ip/n” proportional to a photocurrent Ip flowing across the photodiode PD is outputted from the output point 61b of the current mirror circuit 61. In this case, the electric potential Vm at the output point 61b of the current mirror circuit 61 with respect to ground is “Rm×Im=(Rm/n)×Ip” due to a voltage drop in the resistor Rm, and is proportional to the photocurrent Ip flowing across the photodiode PD. Therefore, a magnitude of the photocurrent Ip flowing across the photodiode PD, i.e. power of the received optical signal, can be found by converting (calibrating) a value Vm of the monitor signal received from the monitoring circuit 60 into “α×Vm” with the use of a conversion coefficient “α”, for example.
However, it is difficult to find power of a burst optical signal for each period of time in which an optical signal is received, on the basis of the monitor signal received from the monitoring circuit 60 illustrated in FIG. 6. This is because, in some cases, the value Vm of the monitor signal cannot reach a steady-state value within a period of time in which an optical signal is received, due to the fact that a rise of the electric potential Vm at the output point 61b of the current mirror circuit 61 is delayed for several μ seconds to several tens of μ seconds with respect to a start point of the period of time in which an optical signal is received (see FIG. 7). This delay is mainly due to a time necessary to turn transistors constituting the current mirror circuit 61 from a nonconductive state into a conductive state.
Therefore, a monitoring circuit 60′ illustrated in FIG. 8 is employed in place of the monitoring circuit 60 illustrated in FIG. 6 by the optical receiver for receiving a burst optical signal, such as an optical receiver of the OLT. The monitoring circuit 60′ illustrated in FIG. 8 is such that the monitoring circuit 60 illustrated in FIG. 6 further includes a load resistor Rb, one end of which is connected to the input point 61a of the current mirror circuit 61, and the other end of which is connected to ground. The monitoring circuit 60′ outputs a monitor signal whose value is an electric potential Vm at the output point 61b of the current mirror circuit 61 in the same manner as the monitoring circuit 60 illustrated in FIG. 6.
In this arrangement, the input point 61a of the current mirror circuit 61 is connected to ground via the load resistor Rb, so that a weak standby current Ib can be kept flowing across the transistors constituting the current mirror circuit 61 even outside a period of time in which an optical signal is received. Because of this, the transistors constituting the current mirror circuit 61 are retained in the conductive state even outside a period of time in which an optical signal is received. This allows the electric potential Vm at the output point 61b of the current mirror circuit 61 to quickly rise at a start point of each period of time in which an optical signal is received (see FIG. 9).
In the arrangement, the input point 61a of the current mirror circuit 61 receives not only the photocurrent Ip flowing across the photodiode PD but also the standby current Ib flowing across the load resistor Rb. It follows that the output point 61b of the current mirror circuit 61 outputs the monitor current Im containing an offset component “Ib/n” corresponding to the standby current Ib. As a result, the electric potential Vm at the output point 61b of the current mirror circuit 61 contains an offset component “(Rm/n)×Ib” corresponding to the standby current Ib. Therefore, in a case where the input point 61a of the current mirror circuit 61 is connected to ground via the load resistor Rb, calculation of a magnitude of the photocurrent Ip flowing across the photodiode PD requires conversion (calibration) of the value Vm of the monitor signal, received from the monitoring circuit 60′, into “α×Vm+β” with the use of conversion coefficients α and β.
Patent Literature 1 discloses an OLT for finding power of a received optical signal by converting a voltage value Vout, which corresponds to the value Vm of the monitor signal described above, into “A×Vout+B+f (Vout)” (see FIG. 1 and Paragraph [0039] of Patent Literature 1). Here, A and B are constants, and f (Vout) is a function indicating a magnitude of an offset current outside the period of time in which an optical signal is received.
Note that the standby current Ib flows across the load resistor Rb constantly regardless of whether or not an optical signal is being inputted, so that the value Vm (t) of the monitor signal contains an offset component regardless of whether or not an optical signal is being received. For this reason, hereinafter, the standby current Ib is also referred to as “offset current”.